The present invention relates to a wavelength division multiplexing optical communication system (WDM optical communication system) and, more specifically, to an optical circuit device using a filter that transmits a lightwave of a desired specific wavelength and reflects lightwaves of desired wavelengths, an optical multiplexer/demultiplexer and an optical transceiver.
A wavelength division multiplexing system (WDM system) includes a plurality of lasers that emit laser beams of different wavelengths, an optical multiplexer that combines the laser beams emitted by the lasers to produce a composite lightwave and introduces the composite lightwave into an optical fiber, an optical demultiplexer that separates the laser beams of the composite lightwave transmitted by the optical fiber, and a plurality of photodiodes. In the WDM system, the capacity of the optical fiber can be easily increased by increasing the number of wavelengths. Recently, the increase of capacity by the WDM system has been applied to low-cost systems, such as local area networks (LANs) and the miniaturization of optical multiplexers and the reduction of cost of multiplexers have been desired.
A generally known conventional optical multiplexer/demultiplexer using a filter that transmits lightwave of a specific wavelength and reflects lightwaves of other wavelengths is comparatively simple in construction and small and can be manufactured at a low cost.
Most generally known methods use filters respectively having different transmission wavelengths, the number of which is equal to that of wavelengths, and a zigzag optical path to separate lightwaves of different wavelengths sequentially. Those methods are disclosed in U.S. Pat. Nos. 5,894,535, 6,198,864 and 6,201,908, and JP-A No. 183741/1999. Other previously proposed methods make lightwaves of wavelength channels fall on a filter at different incident angles. Such methods are disclosed in, for example, U.S. Pat. No. 5,808,763, and JP-A Nos. 49480/1995, 203830/1993 and 48439/1998. Those methods utilize the characteristic of the filter that shifts transmission wavelengths toward the short wavelength side when incident angle xcex8 on the filter increases. Thus, transmission wavelengths can be changed by using different incident angles xcex8 for wavelength channels. Therefore, those methods needs only one filter and hence needs a system simpler in construction than that needed by the former methods.
Accordingly, it is an object of the present invention to provide an optical circuit device that can be miniaturized even if a method that uses lightwaves in a wide wavelength range and makes lightwaves transmitted through wavelength channels fall at different incident angles on a filter.
Another object of the present invention is to manufacture an optical circuit device capable of dealing with lightwaves of wavelengths in a wide working wavelength range at a high productivity.
A representative optical circuit device according to the present invention is an optical multiplexer/demultiplexer. An optical demultiplexer to be described below by way of example is capable of operating also as an optical multiplexer by using its receiving side as a reflecting side. According to one aspect of the present invention, an optical circuit device includes a filter that transmits a lightwave of a desired, specific wavelength and reflects those of other wavelengths, a mirror that reflects the lightwave of the specific wavelength, and a planar lightwave circuit. Usually, the planar lightwave circuit is provided with two grooves in which the filter and the mirror can be inserted.
An optical waveguide included in the planar lightwave circuit has a receiving part (first part) that guides a composite lightwave to the filter at a specific angle, a multipath reflection part (second part) that repeats guiding the lightwave reflected by the filter toward the mirror and reflected again by the mirror toward the filter, and an emitting part (third part) that guides the lightwave transmitted by the filter to an end surface. The lightwave falls on the filter of the multipath reflection part at least two incident angles, and the filter is used as an optical demultiplexer having filters the number of which is less than that of wavelengths.
The multipath reflection part is provided with a bend optical waveguide, and the filter and the mirror are disposed substantially parallel to each other. The substantially parallel arrangement of the filter and the mirror is a significant advantage of the present invention. The bend optical waveguide will be described later.
An optical circuit device representing the present invention can be described in terms of an optical path. An optical circuit device according to the present invention includes a filter that transmits lightwaves of wavelengths in a desired wavelength range and reflects lightwaves of wavelengths in another desired wavelength range, a mirror, a first optical path, a second optical path, and a plurality of third optical paths; wherein the filter is disposed opposite to the first optical path, the filter and the mirror are disposed substantially parallel to each other on the opposite side of the second optical path on a side for receiving the reflected lightwave reflected by the filter, the filter and the mirror are capable of reflecting lightwaves from each other through the second optical path, the third optical paths are disposed so as to correspond to the position of an optical path reaching the filter of the first optical path, and the position of an optical path reaching the filter of the second optical path interposed between the filter and the mirror, the second optical path is formed by a bend optical waveguide, and the second optical path has at least two incident angles on the filter.
The optical circuit device of the present invention can be used as an optical demultiplexer or an optical multiplexer depending on its receiving side.